In recent years, there is performed the study on a communication system called a Heterogeneous Network (HetNet) in which a “pico base station” of which the transmission power and coverage area are small is placed under a “macro base station” of which the transmission power and coverage area are large. The coverage area formed by each base station can be called a cell or a sector in some cases. In the HetNet, because the traffic of the macro base station can be offloaded to the pico base station, the improvement of communication speed can be achieved.
The transmission power of an uplink in user equipment (UE) is determined in accordance with a path loss of the uplink. In other words, the larger the path loss of the uplink is, the larger the transmission power of the uplink becomes. As described above, in the HetNet, the size of the coverage area of the macro base station is different from that of the coverage area of the pico base station. For this reason, a large transmission-power difference is caused between user equipment located at the edge (hereinafter, referred to as “cell edge”) of the coverage area of the pico base station and user equipment located at the cell edge of the macro base station. Therefore, user equipment located within the coverage area of the pico base station receives large interference from user equipment located at the cell edge of the macro base station. Hereinafter, user equipment that is located within the coverage area of a pico base station and is being connected to the pico base station can be referred to as “pico UE” in some cases. Moreover, user equipment that is located within the coverage area of a macro base station and is being connected to the macro base station can be referred to as “macro UE” in some case.
As a technology for reducing such an interference influence, there are reception techniques called IRC (Interference Reject Combining) reception. In the IRC, without using MMSE (Minimum Mean Square Error) reception weight WH (Equation (1)) that uses average interference noise power, an interference influence is reduced by multiplying reception weight WH (Equation (2)) that uses a correlation matrix of an interference signal by a received signal. In Equations (1) and (2), “hp” indicates a channel estimation value between a pico base station and user equipment connected to the pico base station, “σI” indicates average interference noise power, “I” indicates a unit matrix, “RI” indicates a correlation matrix of an interference signal that includes noise power, “yp” indicates a received signal at the pico base station, and “sp” indicates a reference signal of the user equipment connected to the pico base station. Moreover, “XH” indicates the conjugate transpose of a matrix X, and “E[ ]” indicates an ensemble average.WH=hp(hphpH+σII)−1  (1)WH=hp(hphpH+RI)−1 RI=E[(yp−hpsp)(yp−hpsp)]  (2)
Examples of related-art are described in Japanese Laid-open Patent Publication No. 2012-156798
Herein, in the IRC reception, when the sum of the number of desired signals and the number of interference signals is not more than the number of receiving antennas, a beam is directed to a desired signal and NULL is directed to an interference signal so as to be able to avoid interference. Therefore, in a case where the number of interference signals becomes larger than “the number of receiving antennas−the number of desired signals”, interference is clearly hard to be avoided and thus reception quality is degraded. In a pico base station that performs IRC reception in an FDD (Frequency Division Duplex) communication system in which different frequency bands are used in an uplink and a downlink, “the number of desired signals” is equivalent to “the number of transmission streams of pico UE”, and “the number of interference signals” is equivalent to “the number of transmission streams of macro UE”. Therefore, for example, in a case where MU-MIMO (Multiuser Multiple Input Multiple Output) communication is performed between a macro base station and macro UE, the number of interference signals at a pico base station becomes large, and thus the number of interference signals becomes larger than “the number of receiving antennas−the number of desired signals” so as to degrade reception quality in the pico base station.